1. Field of the Invention
The present invention relates generally to a rotary hydraulic valve and more particularly to a rotary hydraulic valve which may be built to operate in an acoustically quiet (non-cavitating) manner.
2. Description of the Prior Art
In certain operating environments, it is desirable to minimize noise wherever possible, particularly in critical components which must be operable at all times. Where hydraulic valves are used in such environments, any cavitation of the operating fluid as it flows through the valves substantially interferes with quiet operation. Accordingly, various efforts have been made to produce servo and throttle valve which can operate hydraulic output members in a non-cavitating manner.
Previous units have been built with linear activating mechanisms. A lapped main spool reciprocates in a linear manner to open or close flow through the passages, thereby effecting changes in the flow rate. Control signals are sent to a solenoid valve or torque motor to actuate the pilot stage which in turn alters the hydraulic balance and/or the position of the main spool. Main spool linear position may be directly or indirectly related to the pilot stage.
One disadvantage of such conventional valves is that leakage flow through the pilot stage introduces hysteresis into valve response. Another disadvantage is that after a control signal is delivered to the servo valve or torque motor, sufficient pressure must build in the pilot stage before the main spool can be actuated, thereby introducing an inherent lag into the control system. Yet another disadvantage is that the small flow passages of the pilot stage, which are needed to effect small movements of the main spool, are very sensitive to fluid contaminants.